additional approaches to fusing the layers, but do not include further

assessment of these here.

ASTER above 60\N, without applying any modifications to deal with

boundary artifacts (Figure \ref{blend-simple}).

\emph{Fused with exponential ramp north of 60\N.}

The first step was to take the pixel-wise difference between SRTM and

SRTM). An exponentially declining fraction of this difference was then

then added back into the ASTER values north of 60\N. This does a fine

job of eliminating the artificial shelf and thus the appearance of a

than SRTM for \Sexpr{round(100*mean(d.delta.vals>0))}\% of pixels (see

Figure \ref{aster-srtm-scatter}). Thus, although adding a constant

offset of \Sexpr{-delta.median} meters to the ASTER DEM would clearly

region), appreciable differences would remain. Figure

\ref{aster-srtm-scatter} also highlights the existence of several

obviously spurious ASTER spikes of >1000m; although not shown here,

Not surprisingly, simple fusion produces an artificial $\sim$12m cliff

in the mean elevation profile (Figure \ref{mean-elevation}). At least in

terms of mean elevation, this artifact is completely removed by both the

transition is, to the eye, slightly smoother in the former case,

although ultimately this would depend on the chosen zone of overlap and

on the exact parameterization of the weighting function.

degree (Figure \ref{mean-slope}). However, the shape of the profile

itself is nearly identical between the two. Although this may partly

reflect inherent SRTM vs ASTER differences, my guess is that CGIAR

some of the high frequency ``noise'' that remains in ASTER?

\textbf{\color{red}[todo: check!]}

expect in the presence of a cliff artifact at the seam.

Interestingly, the aspect layers derived from the two blended DEMs

consistent mean northward inclination at all latitudes in their

respective fusion zones. This pattern is visually obvious at latitudes

between 59.95\N and 60\N in the bottom two panels of Figure

Gaussian weighted averaging produces a layer that exhibits a gradual

transition from ASTER to SRTM, whereas the multiresolution spline yields

an abrupt transition. Not surprisingly, the simple fused layer is even

at the fusion seam itself; note downward (upward) spikes in correlation

(RMSE) at 60\N in the first column of plots in Figures \ref{corr-slope}

and \ref{rmse-slope}.

unexpected (and as-yet unexplained) negative spike at $\sim$59.95\N, the

correlation profiles are fairly well-behaved for both blended layers,

and don't show the same odd behavior as was the case for aspect. Again

transition than does the Gaussian weighted average. The RMSE flow

direction profiles echo this pattern (Figure \ref{rmse-flowdir}), and

indeed look almost the same as those computed using aspect (Figure